I feel like Tien, Yamcha, and Chiaotzu are the 3 obvious choices, but if I take them out of the equation, definitely Gohan. Gohan had such a great character arc early on where we got to see this whiney child grow into a great warrior. Then after Cell he stopped, he got weaker, and it felt like all that potential we were teased about from early on became pointless. Like, he was talked about as having this immense overwhelming hidden potential and power, but after the Cell Saga we got Ultimate Gohan after he was practically forced to continue the fight due to an overwhelming threat. As a character though he just felt so wasted, and there was a lot that could have still been done for Gohan.
I don't fault him for wanting to do school, time of peace, he didn't like fighting, and he was incredibly smart. That's where my issue lies though, for a character that's meant to be so smart, why didn't Gohan keep training with his dad gone. Gohan watched Raditz invade Earth, Vegeta and Nappa invade, Frieza, Androids, Cell, after all of that didn't Gohan stop to think "A lot of dangerous people have come to Earth...with my Father gone...I need to step up to defend the Earth." Even if you make the argument "Oh, but the Earth had Vegeta" Gohan had to save Vegeta from Cell, and it was because of Vegeta that Cell even became perfect to begin with. It never made sense to me that a character as smart as Gohan thought "We'll probably safe" and didn't take precautions.
He's at least "attempting" to do something now, but I kind of wish this was all stuff that was done and thought about earlier, because right now he's my least favorite simply due to all the missed potential that could have been done with him.